<!doctype html>
<!--
     Any copyright is dedicated to the Public Domain.
     http://creativecommons.org/publicdomain/zero/1.0/
-->
<meta charset="utf-8">
<title>CSS Test Reference</title>
<style>
body {
  background: #f3f5f6;
}

div {
  font-size: 15px;
  --width: 401px;
  width: var(--width);
  margin: 20px;
  background: #fff;
  display: flex;
  flex-wrap: wrap;
}

b {
  height: 50px;
  background: red;
  width: calc((var(--width) - 4.5em) / 4); /* .5em gutters */
}

b:not(:last-child) {
  margin-right: 1.5em;
}
</style>
<div><b></b><b></b><b></b><b></b></div>
